The invention relates to a method for reducing the strikethrough effect in inkjet printing on thin substrates, in particular on newsprint.
Today there are already known applications in which inkjet printing methods are used for printing on very thin absorbent substrates such as newsprint. A great disadvantage that occurs here, however, is the so-called strikethrough or strikethrough effect. The substrate may not be able to adequately absorb the amount of ink applied and distribute it on the surface of the substrate so that the ink passes through the substrate, e.g., via fibers in the substrate, and becomes visible on the back side of the substrate and/or print product to a certain extent. This makes two-sided printing with inkjet printing methods on thin substrates such as thin paper much more difficult or even impossible with regard to adequate print quality of the print image to be printed thereby.
For this reason, specially coated substrates and/or papers are used in some applications today to prevent the ink from penetrating too deeply into the substrate. However, this procedure ultimately results in high production costs and/or cost of materials, which is unacceptable especially with a large number of copies, such as that which is customary in the production of newspapers.
The object of the invention is to create an inexpensive method for reducing the strikethrough effect in inkjet printing on thin substrates, in particular newsprint.
According to the invention, a method for reducing the strikethrough effect in inkjet printing on thin substrates, in particular newsprint comprises: determining a normal print quality, determining a normal quantity of ink which would be used for inkjet printing of a certain print image on a strikethrough-resistant substrate on the basis of normal print quality, determining a minimal print quality for the print image to be printed, determining a maximal value for the strikethrough effect, reducing the normal quantity of ink to a reduced quantity of ink, so that the print image has at least the minimal print quality and the maximal value of the strikethrough effect is not exceeded.
With the inventive method, the strikethrough effect, which is perceived as disturbing, can be minimized or even eliminated.
Due to the fact that special paper with a strikethrough-inhibiting coating is not necessary according to this invention, substantial cost savings can be achieved with regard to the substrate, e.g., newsprint.
Another advantage of the inventive method is that by reducing the normal quantity of ink to a reduced quantity of ink, ink savings are possible to a substantial extent and thus the cost of materials for the printing process can be further reduced.
According to a further embodiment of the invention, the normal quantity of ink is defined by an ink drop volume, a geometric coverage of the print image on the substrate and a gray value of the print image, such that the reduction in the normal quantity of ink to the reduced quantity of ink is accomplished by reducing at least one of the following: the ink drop volume, the geometric coverage and the gray value.
This has the advantage that by reducing or eliminating the strikethrough effect and saving on printing ink, the predetermined minimal print quality of the print image can be achieved easily at the same time, i.e., a predetermined print quality for a given printing job, e.g., the quality stipulated by the client, can be achieved.
According to another embodiment of the invention, to determine the normal quantity of ink, a first ink drop volume corresponding to the normal quality is determined and the reduction in the normal quantity of ink to the reduced quantity of ink is achieved by reducing the first ink drop volume to a second smaller ink drop volume.
It has been recognized according to the present invention that when the ink drop is assumed to be an ideal sphere geometrically, the diameter of the ink drop is reduced to a far lesser extent when the ink drop volume is reduced by a certain percentage. Thus, with uniform resolution of a print image, i.e., with a uniform spacing of ink drops apart from one another and a reduction in the ink drop volume, the geometric coverage of the print image is reduced only slightly.
The user of the inventive method thus has sufficient access to a manipulated variable, i.e., the ink drop volume to be reduced, to reduce any strikethrough effect.
If necessary, the user may additionally increase the resolution of the print image to increase the geometric coverage and thus the quality of the print image.
According to yet another embodiment of the invention, the first ink drop volume is reduced by approximately 50% to approximately 75%.
According to a further embodiment of the invention, the first ink drop volume is determined to be less than 100 pL.
According to an alternative further embodiment of the invention, the first ink drop volume is determined to be approximately 100 pL and the second ink drop volume is determined to be approximately 50 pL to approximately 25 pL.
According to yet another further embodiment of the invention, a resolution corresponding to normal print quality is set at approximately 300 dpi for the print image.
According to an alternative further embodiment of the invention, the resolution for the print image corresponding to normal print quality is set at more than 300 dpi.
According to another embodiment of the invention, the normal quantity of ink is reduced to a reduced quantity of ink by reducing the geometric coverage of the print image on the substrate.
With this embodiment of the invention, which means optimization of the print image to the extent that with a reduced quantity of ink there is only a slight reduction in the print image quality, it is likewise possible in a simple and inexpensive manner to reduce and/or prevent a strikethrough effect to a sufficient extent.
According to a further embodiment of the invention, to determine the normal quantity of ink, tone values are defined for different regions of the print image corresponding to normal quality and the geometric coverage is reduced by defining a transfer function or gradation curve so that at least a portion of the output values of the transfer function or gradation curve is smaller than the respective input values of the transfer function or gradation curve, and by varying the defined tone values of the print image by using the tone values as input values for the transfer function or gradation curve and defining the output values of the transfer function or gradation curve as new tone values for the print image.
In other words, in this embodiment of the inventive method, the normal quantity of ink is reduced to the reduced quantity of ink by converting, i.e., recalculating, the input tone values of a print image, i.e., tone values for inkjet printing of the print image on a strikethrough-resistant substrate by means of the transfer function or gradation curve to output tone values that correspond to the minimal print quality.
According to yet another embodiment of the invention, the transfer function or gradation curve is defined so that tone values of the print image remain unchanged in the light range and tone values of the print image are reduced near a full tone.
According to another embodiment of the invention, to determine the normal quantity of ink, a raster image is printed for the print image and the geometric coverage is reduced by forming a bit-oriented or pixel-oriented mask, whose raster structure is reduced quantitatively in comparison with that of the raster image, and mapping the raster image through the bit-oriented or pixel-oriented mask so as to form a reduced raster image.
In other words, a print image, e.g., a fully ripped bit map, i.e., a finished raster image, is “thinned out” by means of the bit-oriented or pixel-oriented mask which may be hardware-based or software-based, for example, through a logic link with this mask, so that the strikethrough effect is reduced or minimized and the minimal print quality is achieved.
According to a further embodiment of the invention, the raster structure of the bit-oriented or pixel-oriented mask is embodied so that it is 50% reduced quantitatively in comparison with that of the raster image.
According to yet another embodiment of the invention, for determining the normal quantity of ink, a raster image of the print image is defined and the geometric coverage is reduced by removing a certain number of pixels in contiguous areas of the raster image so as to form a reduced raster image.
This embodiment of the invention is advantageous in particular with repeating print images because a print image with a reduced number of pixels accordingly, in which the quantity of ink is reduced and the minimal print quality is met, can be reused repeatedly for printing.
According to a further embodiment of the invention, for inkjet printing of text with character sets, i.e., fonts provided for this purpose, a certain number of pixels are removed, so that the respective contours of the characters and/or letters are retained.
In other words, pixels are removed in a targeted manner so that the contour of the individual letters is maintained, such that in places where the strikethrough effect is manifested in particular, namely in the large contiguous areas of characters, matrix dots are removed in a targeted manner, so that there are no obvious gaps between the matrix dots.
According to another embodiment of the invention, the reduction in the normal quantity of ink to the reduced quantity of ink is achieved by reducing the gray value of the print image.
According to this embodiment of the invention, it is likewise possible in a simple manner to reduce the quantity of ink applied, so that the strikethrough effect is reduced and/or prevented and nevertheless the minimal print quality is achieved.
According to a further embodiment of the invention, fonts having a gray value of less than 100% are defined for inkjet printing of text.
In other words, text character sets and/or text fonts that are normally predefined in full color tones are defined according to the present invention with a gray value of less than 100%, so that the geometric coverage of the respective print image, i.e., of the individual characters, is reduced.
The present invention is described in greater detail below on the basis of preferred embodiments and with reference to the accompanying figures.